Support cue files

[simmel]

Would be nice, and so does these people think, if mopidy could support .cue files. They are basically the same as .m3u files except that they are sections of a file instead of multiple files. See https://en.wikipedia.org/wiki/Cue_sheet_%28computing%29

Ideally .cue files would expand into a folder in Local media.

[adamcik]

This is a bit tricky, as we don't currently have a way to do "bookmarks" into a file like we would need for the cues. I presume we should also be ignoring the file and replacing it with the queue items.

Anyway, the main challenge here is updating our core data models and APIs to work for this type of use. Assuming we get this sorted somehow it would also allow for things like annotations in Soudcloud type streams.

This might also play in to #620, but I haven't thought that through just yet.